When two creatures fight, they each deal damage to each other at the same time.
And any damage from a source with Wither (or Infect) dealt to a creature is dealt in the form of -1/-1 counters.
So if your Necroskitter were to fight your opponent's 3/3, then both would live. But the 3/3 would now have a -1/-1 counter on it, making it a 2/2.
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That's awesome artifact for Xantcha deck.